Protective coatings for alloys

Abstract

Aluminium-containing alloys are protected against high temperature corrosive attack by providing an amorphous silica coating, e.g. by plasma assisted vapor deposition, followed by heat treating under conditions such that a coherent, protective alumina coating or layer is formed on the surface of the alloy in place of the silica coating. The silica protects the alloy from corrosion initially and then acts as an oxidant source so that aluminium from the alloy can react to form the alumina coating. The alloy may be a superalloy such as a nickel-base superalloy.

I claim: 
     
       1. A method of providing an aluminum-containing nickel-base superalloy with a protective surface coating substantially of alumina which comprises (i) coating a surface of the alloy with an adherent, amorphous coating of silica by a vapour depositing coating technique; and   (ii) heat treating the coated alloy at a temperature greater than 800° C. under conditions such that a coherent, protective alumina coating is formed on the surface of the alloy.   
     
     
       2. A method according to claim 1 wherein the aluminium content of the alloy is such that the alloy is incapable of forming a protective alumina coating when heated in a gaseous oxidising atmosphere. 
     
     
       3. A method according to claim 1 wherein the coating of silica is provided by plasma activated vapour deposition.